The invention relates to a process for single-stage postcondensation of polycondensates, especially polyamide 6.6 granulate, in a fluidized bed reactor using nitrogen (N.sub.2) as the carrier gas.
For producing filaments of polycondensates for industrial yarns, a thermal posttreatment in the solid state, generally known as postcondensation, is required in order to adjust the degree of polymerization. In the postcondensation of polyamide 6.6, the polyamide is typically treated with heat in a vacuum in granulate form, or an inert gas, as a rule dry nitrogen, is passed through the granulate. Some of the inert gas, laden with moisture and contaminants, is blown off after passing through the granulate bed. Dessicators, such as deep-freeze traps, silica gel, or molecular sieves, are often located in the inert-gas loop in order to dry the polymers. All the known processes for postcondensation of polyamide 66 require very long postcondensation times, as a rule more than 20 hours.
In postcondensation in the solid state, as a rule one begins with granulate that is not completely dry. Adjusting the moisture in a polyamide granulate by adding water is known. For instance, in the process of European Patent Disclosure EP-A 0 092 898, postcondensation is first done in at least two stages, and then in a conditioning phase a regulated quantity of water is added to the polyamide immediately before the melting in the extruder. With the known process, the water content in the polymer is meant to be adjusted as accurately as possible before the extrusion. However, adding water in metered fashion to the cooled granulate or to the hot extruder has the disadvantage that the metered dosage can fluctuate, or undesired temperature fluctuations can occur. As a rule, fractional streams pass from the extruder to a plurality of stations whose operation has to be stopped, for instance for changing spinning nozzles, thus necessitating a complicated adaptation of the metered addition of water. Both manual addition and metering using a pump are too inaccurate to prevent deviations in the quality of the resulting filament.